Scintillators have found widespread usage for the detection of neutron radiation, as emitted by many radioactive sources. Liquid scintillators loaded with neutron absorbers generally possess a better pulse shape discrimination (PSD) than solid scintillators, and thus, are preferred when discrimination between neutron and gamma events are required.
Typically, isotopes with high cross sections, such as 10B, 6Li, and Gd, are used for neutron capture reactions. The energy emitted by the neutron capture reaction is generally made visible by the inclusion of a fluorophore that absorbs the energy and emits it as visible light. There is also the possibility that the neutron absorber also functions as a fluorophore.
Lithium-6 doped scintillators are particularly attractive due to the large Q value (i.e., 4.8 MeV) and high photon yield released as a result of the lithium-6 producing an alpha (4He) and tritium (3H) particle along with emitted energy. Lithium-6 doped scintillators also advantageously do not generate secondary gamma rays.
However, there are at least two properties of current lithium-loaded liquid scintillators that significantly limit their use. These properties include a generally lower than optimum light output and a lower than optimum PSD for distinguishing neutron events from gamma events. The most significant obstacle in improving these characteristics of lithium-loaded liquid scintillators has been the very low loading capability of lithium compounds (e.g., lithium salicylate and lithium propionate) in non-polar solvents (e.g., toluene and xylene). Although solid neutron scintillators with high lithium content are available, the lack of discrimination against gamma radiation limits their applications.
Numerous efforts have been undertaken to increase the lithium loading in liquid scintillators. However, these efforts have met with little success. For example, efforts to increase the lithium loading by incorporating a higher weight percentage of the lithium compound in a non-polar liquid generally results in clouding of the liquid. Clouding of the liquid significantly reduces the light output and PSD characteristics of the liquid scintillator. The liquid needs to be substantially transparent in order for the scintillator device to function in an effective manner. Other efforts include incorporating a more polar solvent (e.g., methanol or water) into the liquid scintillator in order to increase the lithium loading since the lithium salts tend to be significantly more soluble in water and other polar solvents than in non-polar solvents. However, this methodology has generally resulted in poor light output and PSD characteristics because of the adverse effects, such as quenching of scintillation, of polar solvents in liquid scintillators.
Accordingly, there is a need for new liquid scintillator compositions with significantly higher lithium loading in non-polar solvents while remaining substantially transparent (preferably, completely transparent, i.e., non-turbid) in these solvents. Such liquid scintillator compositions would advantageously possess significantly higher light outputs and PSD characteristics.